In the production of data storage media, e.g., optical discs, magnetic discs, magneto-optical discs, and the like, such as compact discs (CDs) and digital video discs (DVDs), surface features are created on the disc during an injection molding process. These surface features are typically locators (e.g., pits or grooves) produced by molding a plastic material against a stamper that contains a negative image of the features to be molded. Stampers are typically made by electroplating nickel against a master produced by conventional photolithographic techniques.
Various types of molds have long been in use for preparing optical discs from thermoplastic resins. Molds for these purposes are typically manufactured from metal or a similar material having high thermal conductivity. For most purposes, high thermal conductivity is desirable since it permits the resin in the mold to cool rapidly, shortening the molding cycle time. At times, however, cooling is so rapid that the resin freezes instantaneously at the mold surface upon introduction into the mold, forming a thin solid layer which, especially if is contains a filler, can create rough surfaces, voids, porosity and high levels or residual stress and orientation. In an optical disc, for example, such imperfections impede the optical properties and decrease or eliminate of the performance of the optical disc.
Therefore, in an injection molding of compact discs, for audio, video, or computer data storage and retrieval applications, heat transfer through the mold has a strong effect on molding time and disc attributes such as birefringence, flatness, and accuracy of feature replication.
One method for affecting heat transfer and improving the cycle time during injection molding is known as the technique of managed heat transfer (MHT). The basic principle of managed heat transfer is applying a passive thermal insulating layer to the mold to control the transient heat transfer between molten resin materials and the mold surfaces during the injection molding. The insulating layer comprises materials having both low thermal diffusivity and conductivity, thus slowing the cooling of the molded resin, and good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. For improving mechanical strength, strong adhesion to the insulating layer, abrasion resistance, oxidation resistance and to better control thermal impedance, at least one skin layer may be bonded to the insulating layer.